Keep track of which field names are in ignored source. #130931
Description
In the world where almost all fields are unmapped it is important to know if an unmapped field exists in order to avoid needlessly reading and parsing ignored source. Much fields will be sparse (a field not existing in all documents).
In order to speed this up, we can explore creating an inverted index on top of ignored source. So that we can skip loading stored fields for document that will not have the requested field.